Fuel cells are devices that directly convert chemical energy of reactants, i.e., fuel and oxidant, into direct current (DC) electricity. For a number of applications, fuel cells can be more efficient than conventional power generation, such as combustion of fossil fuel, as well as portable power storage, such as lithium-ion batteries.
In general, fuel cell technology includes a variety of different fuel cells, such as alkali fuel cells, polymer electrolyte fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells and enzyme fuel cells. Today's more important fuel cells can be divided into several general categories, namely (i) fuel cells utilizing compressed hydrogen (H2) as fuel; (ii) proton exchange membrane (PEM) fuel cells that use alcohols, e.g., methanol (CH3OH), metal hydrides, e.g., sodium borohydride (NaBH4), hydrocarbons, or other fuels reformed into hydrogen fuel; (iii) PEM fuel cells that can consume non-hydrogen fuel directly or direct oxidation fuel cells; and (iv) solid oxide fuel cells (SOFC) that directly convert hydrocarbon fuels to electricity at high temperature.
Compressed hydrogen is generally kept under high pressure and is therefore difficult to handle. Furthermore, large storage tanks are typically required and cannot be made sufficiently small for consumer electronic devices. Conventional reformat fuel cells require reformers and other vaporization and auxiliary systems to convert fuels to hydrogen to react with oxidant in the fuel cell. Recent advances make reformer or reformat fuel cells promising for consumer electronic devices. The most common direct oxidation fuel cells are direct methanol fuel cells or DMFC. Other direct oxidation fuel cells include direct ethanol fuel cells and direct tetramethyl orthocarbonate fuel cells. DMFC, where methanol is reacted directly with oxidant in the fuel cell, has promising power application for consumer electronic devices. SOFC convert hydrocarbon fuels, such as butane, at high heat to produce electricity. SOFC requires relatively high temperatures in the range of 1000° C. for the fuel cell reaction to occur.
Another type of liquid fuel is hydrazine, which can be anhydrous or in its monohydrate form. Hydrazine is soluble in water and decomposes to form hydrogen in the presence of water, as follows:N2H4H2O+H2O→2H2+N2+2H2O
The chemical reactions that produce electricity are different for each type of fuel cell. For DMFC, the chemical-electrical reaction at each electrode and the overall reaction for a direct methanol fuel cell are described as follows:
Half-reaction at the anode:CH3OH+H2O→CO2+6H++6e−
Half-reaction at the cathode:1.5O2+6H++6e−→3H2O
The overall fuel cell reaction:CH3OH+1.5O2→CO2+2H2O
Due to the migration of the hydrogen ions (H+) through the PEM from the anode to the cathode and due to the inability of the free electrons (e−) to pass through the PEM, the electrons flow through an external circuit, thereby producing an electrical current through the external circuit. The external circuit may be used to power many useful consumer electronic devices, such as mobile or cell phones, calculators, personal digital assistants, laptop computers, and power tools, among others.
DMFC is discussed in U.S. Pat. Nos. 5,992,008 and 5,945,231, which are incorporated by reference herein in their entireties. Generally, the PEM is made from a polymer, such as Nafion® available from DuPont, which is a perfluorinated sulfonic acid polymer having a thickness in the range of about 0.05 mm to about 0.50 mm, or other suitable membrane. The anode is typically made from a Teflonized carbon paper support with a thin layer of catalyst, such as platinum-ruthenium, deposited thereon. The cathode is typically a gas diffusion electrode in which platinum particles are bonded to one side of the membrane.
In another direct oxidation fuel cell, borohydride fuel cell (DBFC) reacts as follows:
Half-reaction at the anode:BH4−+8OH−→BO2−+6H2O+8e−
Half-reaction at the cathode:2O2+4H2O+8e−8OH−
Chemical metal hydride fuels are promising due to their relatively higher energy density, i.e., amount of hydrogen per mass or volume of fuel. In a chemical metal hydride fuel cell, sodium borohydride is reformed and reacts as follows:NaBH4+2H2O→(heat or catalyst)→4(H2)+(NaBO2)
Half-reaction at the anode:H2→2H++2e−
Half-reaction at the cathode:2(2H++2e−)+O2→2H2O
Suitable catalysts for this reaction include platinum, ruthenium, and other metals. The hydrogen fuel produced from reforming sodium borohydride is reacted in the fuel cell with an oxidant, such as O2, to create electricity (or a flow of electrons) and water byproduct. Sodium borate (NaBO2) byproduct is also produced by the reforming process. A sodium borohydride fuel cell is discussed in U.S. Pat. No. 4,261,956, which is incorporated by reference herein in its entirety.
Despite the potential benefits of higher energy density, chemical metal hydride fuels have not achieved the desired energy density for use with portable electronic devices including the amount of hydrogen that can be released from the fuel. One of the reasons for this in sodium borohydride fuel cells is that, in practice, substantially more water is needed to realize complete oxidation of all of the solid sodium borohydride than stoichiometry would indicate. Hence, there remains a need to increase the energy density and maximize the release of hydrogen from chemical metal hydride fuels.